Achieving ignition on the National Ignition Facility required many diagnostics to probe the optical, x-ray, and nuclear performance of a wide range of inertial confinement fusion implosions and other physics measurements.
(Download Image)Diagnostics Explored the ‘Unknowns’ in NIF Ignition Experiments
Sophisticated diagnostic equipment played a central role in testing the inertial confinement fusion (ICF) models, simulations, and experimental designs used in the National Ignition Facility (NIF) laser shots that led to ignition. That role was described in a recent Physics of Plasmas paper that traced the evolution and contributions of diagnostics to ICF experiments over the past decade.
In the paper, Lawrence Livermore National Laboratory (LLNL) researchers and their colleagues on the National Diagnostic Working Group show how diagnostics were used to measure the experimental results and explore the “known unknowns” and “unknown unknowns” of the complex physics involved in using lasers to squeeze isotopes of hydrogen atoms until they fuse and trigger a self-sustaining fusion reaction leading to ignition.
In the 1950s, science philosopher Karl Popper famously described experimental falsifiability as a key test of a physical theory. At NIF, data acquired from the continuous improvement of diagnostics was able to falsify early theories for ignition and aid in the development of new theories.
“The proximate role of diagnostics in a mission-driven activity such as ICF is to measure observables in order to falsify or validate theory that is embodied in simulations,” the researchers said.
Improving Theory
“Falsifiability is the root reason we do experiments—to prove theory wrong and suggest how to improve it,” said lead author Joe Kilkenny, vice president for High Energy Density Physics at General Atomics, a partner in the NIF ignition enterprise.
Ignition on NIF occurs when the fusion energy from an ICF implosion exceeds the energy delivered by the lasers to the target (see “How NIF Works”). Ignition is an essential aspect of NIF’s primary mission to support the National Nuclear Security Administration’s science-based Stockpile Stewardship Program.
In the Physics of Plasmas paper, the researchers noted that NIF’s early design work recognized that the extreme precision required for ignition would require fine-tuning by using experiments to measure and adjust known unknowns.
Experimental results are used to test codes against the observables; this leads to adjustments to the models or the boundary or initial conditions of the experiments, “such as the beam balance, time history, wavelength difference, or target quality, which hopefully brings models and experiment closer together,” they said. “This process is more appropriately described as micro-falsification, or micro-validation.”
NIF employs more than 100 diagnostics to monitor experimental performance and identify degradation mechanisms and mitigations. A “core” set measures several key theoretical parameters: the shape, or symmetry, of the implosion; the adiabat, or resistance to compression, of the fusion fuel; the implosion’s velocity; and the amount of target capsule material mixing with the fuel.
“Achieving ignition on NIF has required many types of experiments with this core set of diagnostics,” the researchers said, “some constraining known unknowns and some revealing surprises—unknown unknowns.”
Examples of unknown unknowns included the formation of “ice” on the laser entrance hole window, which affected laser burn-in time, and the surprising observation of a significant “drift” velocity of the hotspot at the center of the target capsule in a burning plasma.
Decades of Effort
The researchers noted that LLNL’s success in achieving ignition on NIF, and the development of the diagnostics that were crucial to all phases of getting to ignition, was a multi-decade activity involving hundreds of scientists and engineers from many institutions. The diagnostic effort was coordinated by the National Diagnostic Working Group.
“Over the years,” noted Kilkenny, “we made major improvements in our measurement capability, particularly in x-ray and neutron imaging, allowing us to make the necessary fixes for the degrading factors and guiding us to demonstrate ignition and burn in the laboratory.”
“This paper underlines the critical contribution of NIF’s diagnostics to the achievement of ignition,” added Andrew MacKinnon, NIF National Diagnostics program leader, “and is a great summary of the decades of effort by the national and international diagnostic community to develop these measurement capabilities for High Energy Density science on NIF.”
Joining Kilkenny and MacKinnon on the paper were LLNL researchers Art Pak, Nino Landen, Vladimir Smalyuk, Alastair Moore, Nathan Meezan, Steve Haan, Warren Hsing, and Dave Bradley; Steve Batha of Los Alamos National Laboratory; Maria Gatu-Johnson of the Massachusetts Institute of Technology; and Sean Regan of the Laboratory for Laser Energetics at the University of Rochester.
More Information:
“The crucial role of diagnostics in achieving ignition on the National Ignition Facility (NIF),” Physics of Plasmas, August 14, 2024
“A decades-long journey to ignition,” Scilights, August 16, 2024
“National Diagnostic Working Group (NDWG) for inertial confinement fusion (ICF)/high-energy density (HED) science: The whole exceeds the sum of its parts,” Review of Scientific Instruments, August 22, 2023
“Diagnostics Were Crucial to LLNL’s Historic Ignition Shot,” NIF & Photon Science News, April 12, 2023
“NIF Diagnostics Played Key Role in Fusion Milestone,” NIF & Photon Science News, November 8, 2021
“Nuclear Diagnostics Help Pave the Way to Ignition on NIF,” NIF & Photon Science News, March 6, 2020
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